Towards permanent seismological monitoring in the Dutch North Sea: Progress and early results

The North Sea is transitioning from a hydrocarbon province to a wind-turbine and CO2-sequestration hotspot. The latter activity needs assurance that the CO2 is kept within the envisioned subsurface containers. Microseismic monitoring is one of the methods to track the movement of the injected CO2. Additionally, the North Sea experiences both tectonic earthquakes and events that are related to gas production. To sufficiently detect, locate and characterize the different events, onshore sensors do not suffice. The seismic network thus needs to be expanded into the sea. The dynamic marine environment, characterized by shallow water, active sand dynamics, and diverse marine life, makes it unfavourable for standard seismological deployments.

In this work, we report a series of experiments toward deploying a seismometer in the Dutch North Sea. The experiments were conducted at shallow water depth (~2m) near the NIOZ harbour at Texel Island and at ~10m depth in the Wadden Sea. We have chosen to deploy a broadband seismometer, so that the acquired data is not only useful for local monitoring, but also for crustal studies using teleseismic earthquakes and teleseisms. We tested two designs, one with a sea-bottom seismometer and the other with a posthole one. Furthermore, we developed a full prototype for a stand-alone station setup, which has been tested in the Wadden Sea. The sea-bottom seismometer performed well at long periods > 10s, while the posthole version showed a higher signal-to-noise ratio at shorter periods, making it more stable for local seismicity detection and localisation. Due to active sand dynamics, the sea-bottom sensor showed a higher temporal variation with the sensors' masses, requiring mass balancing more frequently than the posthole version. The posthole sensor remained clean, whereas the sea-bottom sensor acted as a reef for all kinds of marine life.